Issue 6787 - Apply rewrite filter even if the rewrite URL is blank

lib/requestBlocker.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 176 matching lines @@
   let [docDomain, sitekey, specificOnly] = getDocumentInfo(page, frame,
                                                            originUrl);
   let [filter, thirdParty] = matchRequest(url, type, docDomain,
                                           sitekey, specificOnly);
 
   let result;
   let rewrittenUrl;
 
   if (filter instanceof BlockingFilter)
   {
-    if (filter.rewrite)
+    if (typeof filter.rewrite == "string")
     {
       rewrittenUrl = filter.rewriteUrl(details.url);
       // If no rewrite happened (error, different origin), we'll
       // return undefined in order to avoid an "infinite" loop.
       if (rewrittenUrl != details.url)
         result = {redirectUrl: rewrittenUrl};
     }
     else
       result = {cancel: true};
   }
@@ skipping 120 matching lines @@
 }
 
 FilterNotifier.on("subscription.added", onFilterChange);
 FilterNotifier.on("subscription.removed", onFilterChange);
 FilterNotifier.on("subscription.updated", onFilterChange);
 FilterNotifier.on("subscription.disabled", arg => onFilterChange(arg, true));
 FilterNotifier.on("filter.added", onFilterChange);
 FilterNotifier.on("filter.removed", onFilterChange);
 FilterNotifier.on("filter.disabled", arg => onFilterChange(arg, true));
 FilterNotifier.on("load", onFilterChange);